Conventional hardness testing devices indicate the hardness of a metal or other material by the depth of penetration achieved by a penetrating probe under a given load or force. The depth serves as a numerical index to the hardness of the material. In certain instances, other physical dimensions of an impression, such as its diameter serve to indicate the hardness of the material. Alternatively, a numerical index to the hardness may be provided by measuring the load or force necessary to effect a desired depth of penetration.
Regardless of the type of device used, in calibrating the device for hardness, several readings or measurements are usually taken on a standard sample. Any variations between such readings are generally accountable to inaccuracies in the testing device itself rather than to inhomogeneities in the specimen. In other words, friction, initial impacts between the penetrator and specimen, and environmental vibrations transmitted through the testing device supports, all contribute towards the inability of the device to yield consistent readings.
As a result of the foregoing difficulties, hardness values for specific materials are given as a numerical range varying between the lowest and highest reading obtained on any one specimen. The numerical differences between the lower and higher numbers in the range are oftentimes sufficient to cause an overlapping of ranges for materials of similar hardness, whereby a differentiation between such materials is extremely difficult.
In my U.S. Pat. No. 2,839,917 issued June 24, 1958, I disclose an improved hardness testing device or machine which overcomes many of the problems contributing towards inaccuracy in readings. For example, many prior art hardness testers employ dead weights in providing a force or load on the penetrating probe. Clearly any outside vibrations causing motion of the hardness testing device itself will establish inertial forces through the dead weights which can impair the accuracy of penetration measurements. This specific problem is overcome by avoiding the use of dead weights and providing a force applying means such as a fluid operated bellows, all as described in my referred to U.S. patent.
Notwithstanding improved versions of hardness testers such as exemplified by my referred to patent, in all such devices which incorporate a frame body with spaced opposed portions between which forces are applied in urging the penetrator against a specimen, the frame body structure itself is subjected to stresses which can result in slight "springing" of the frame, thus changing the reference position of the specimen where it is supported on a portion of the frame caused to "give" or move. As a result, measurements of actual penetration of a probe under load urged against the specimen will be inaccurate since movement of the frame itself will result in movement of the specimen relative to the probe. Where very thin specimens are being tested, the depth of penetration is necessarily extremely slight, the same being measured in thousandths of an inch. It will be readily apparent that no matter how rigid the body frame is constructed, there still results a slight "give" or "springing" of the frame under loads particularly when extremely hard material is being tested.